The present invention relates to an elastic surface wave device and, more particularly, to an elastic surface filter used as a frequency-selective filter.
An elastic surface wave filter is generally constructed as shown in FIG. 1 wherein a piezoelectric substrate 4 is adhered to the surface of a stem 1, and an input electrode 2 and an output electrode 3 are formed in a comb shape on the surface of the piezoelectric substrate 4 to oppose each other at a predetermined distance. Stem leads 5 extending through the stem 1 are bonded by wires 6 to these electrodes on the substrate, and a shell 7 covers the upper surface of the stem 1.
When such a surface wave filter operates, an electric signal is converted into an elastic surface wave by the input electrode, it propagates along the piezoelectric substrate towards the output electrode, and an electric signal is obtained from the output electrode. In this case, the surface wave propagates on the surface of the substrate. Further, an unnecessary bulk wave that propagates inside the substrate is generated.
The characteristics of the surface wave filter may be improved by eliminating the bulk wave as much as possible, and various methods have been proposed for suppressing the bulk wave. For example, it has been proposed to roughen the bottom surface of the piezoelectric substrate (surface on which the electrodes are not formed) by mechanical means or the like, or to form grooves thereon. It is also possible to make the piezoelectric substrate thicker to absorb and suppress the bulk wave. All these methods adopt a structure for absorbing the bulk wave inside the substrate, and the processing precision of the piezoelectric substrate contributes to the suppressing effects of the bulk wave.
However, with these methods, cracks in the piezoelectric substrate tend to occur more frequently, much labor is required for obtaining a roughened surface, and the variation in the yield is significant. The thickened piezoelectric substrate also leads to an increase in the manufacturing cost which is particularly disadvantageous for mass production.